As generally known in the art, power steering apparatuses for automobiles include a hydraulic power steering apparatus utilizing hydraulic pressure of a hydraulic pump, which has been used since its initial introduction, and an electric power steering apparatus utilizing an electric motor, use of which has been gradually universalized since the 1990's.
In the existing hydraulic power steering apparatus, a hydraulic pump, which is a power source for supplying steering power, is driven by an engine, which causes the hydraulic pump to continuously consume energy regardless of whether or not the steering wheel is being rotated. In the electric power steering apparatus, when steering torque is generated by rotation of a steering wheel, a motor supplies steering power in proportion to the generated steering torque. Therefore, in energy efficiency terms, the electric power steering apparatus is more advantageous than the hydraulic power steering apparatus.
FIG. 1 illustrates the construction of a conventional electric power steering apparatus.
As shown in FIG. 1, a conventional electric power steering apparatus for an automobile includes a steering system 100, which includes elements from a steering wheel 101 to both wheels 108, and a steering power mechanism 120 for supplying steering power to the steering system 100.
The steering system 100 includes a steering shaft 102 having an upper end connected to the steering wheel 101, so that the steering shaft 102 rotates together with the steering wheel 101, and a lower end connected to a pinion shaft 104 via a pair of universal joints 103. The pinion shaft 104 is connected to a rack bar 109 via a rack-pinion mechanism 105. Both ends of the rack bar 109 are connected to the wheels 108 via tie rods 106 and knuckle arms 107.
The rack-pinion mechanism 105 includes a pinion gear 111 formed on the lower end of the pinion shaft 104 and a rack gear 112 formed on a side of the outer peripheral surface of the rack bar 109 to engage with the pinion gear 111. The rack-pinion mechanism 105 converts the rotational motion of the pinion shaft 104 into a linear motion of the rack bar 109. Particularly, when the driver operates the steering wheel 101, the pinion shaft 104 rotates accordingly. The rotation of the pinion shaft 104 causes the rack bar 109 to move linearly in the shaft direction. The linear motion of the rack bar 109 is transmitted to and operates the wheels 108 via the tie rods 106 and the knuckle arms 107.
The steering power mechanism 120 includes a torque sensor 121 for sensing steering torque applied to the steering wheel 101 by the driver and outputting an electric signal in proportion to the sensed steering torque, a velocity sensor 122 for sensing the velocity of the automobile and outputting an electrical signal, an ECU (electronic control unit) 123 for generating a control signal based on the electric signals from the torque sensor 121 and the velocity sensor 122, and a motor 130 for generating steering power based on the control signal from the ECU 123.
The electric power steering apparatus is operated as follows: when the driving wheel 101 is rotated, driving torque is generated and transmitted to the rack bar 109 via the rack-pinion mechanism 105. In addition, the generated steering torque causes the motor 130 to generate steering power, which is transmitted to the rack bar 109. As such, the steering torque generated by the steering system 100 is combined with the steering power generated by the motor 130, so that the rack bar 109 is moved in the shaft direction.
FIG. 2 is an exploded perspective view briefly showing an electronic control device according to the prior art.
As shown in FIG. 2, the electronic control device according to the prior art includes a PCB 201, a metal substrate 211, and a radiation plate 221.
The PCB 201 has various components mounted thereon, including a capacitor 203 for absorbing ripple current included in current supplied to a motor; a shunt resistor 205 for sensing the current supplied to the motor; an FET (field effect transistor) 207 for switching the current supplied to the motor based on the size and direction of steering power; a coil 209 for removing electromagnetic noise; and a microcomputer 210 for calculating steering power based on steering torque and the automobile's velocity.
The metal substrate 211 is spaced a predetermined distance from the PCB 201, and a body 208 of the FET 207 is coupled to the upper surface of the metal substrate 211.
The radiation plate 221 is positioned on the bottom surface of the metal substrate 211 and is coupled to the PCB 201 and the metal substrate 211 with bolts 213, in order to radiate heat, which is generated from the body 208 of the FET 207 and transmitted via the metal substrate 211, into the air.
However, the conventional electronic control device, constructed as above, has a problem in that it cannot effectively radiate a large quantity of heat, which is generated by the body 208 of the FET 207 when current necessary to drive the motor flows, because the metal substrate 211 and the radiation plate 221 are stacked on each other.
Any attempt to increase the thickness of the radiation plate 221, in order to improve heat radiation, is limited by spatial restrictions.
When a larger number of FETs 207 are used, their assembly process becomes complicated, because bolts 213 are used to couple respective bodies 208 of the FETs 207 to the metal substrate 211.